Re: [RFC PATCH v4 0/4] vfio-ccw: Fix interrupt handling for HALT/CLEAR

[Halil Pasic <pasic@linux.ibm.com>]

On Tue, 13 Apr 2021 20:24:06 +0200
Eric Farman <farman@linux.ibm.com> wrote:

> Hi Conny, Halil,
> 
> Let's restart our discussion about the collision between interrupts for
> START SUBCHANNEL and HALT/CLEAR SUBCHANNEL. It's been a quarter million
> minutes (give or take), so here is the problematic scenario again:
> 
> 	CPU 1			CPU 2
>  1	CLEAR SUBCHANNEL
>  2	fsm_irq()
>  3				START SUBCHANNEL
>  4	vfio_ccw_sch_io_todo()
>  5				fsm_irq()
>  6				vfio_ccw_sch_io_todo()
> 
> From the channel subsystem's point of view the CLEAR SUBCHANNEL (step 1)
> is complete once step 2 is called, as the Interrupt Response Block (IRB)
> has been presented and the TEST SUBCHANNEL was driven by the cio layer.
> Thus, the START SUBCHANNEL (step 3) is submitted [1] and gets a cc=0 to
> indicate the I/O was accepted. However, step 2 stacks the bulk of the
> actual work onto a workqueue for when the subchannel lock is NOT held,
> and is unqueued at step 4. That code misidentifies the data in the IRB
> as being associated with the newly active I/O, and may release memory
> that is actively in use by the channel subsystem and/or device. Eww.
> 
> In this version...
> 
> Patch 1 and 2 are defensive checks. Patch 2 was part of v3 [2], but I
> would love a better option here to guard between steps 2 and 4.
> 
> Patch 3 is a subset of the removal of the CP_PENDING FSM state in v3.
> I've obviously gone away from this idea, but I thought this piece is
> still valuable.
> 
> Patch 4 collapses the code on the interrupt path so that changes to
> the FSM state and the channel_program struct are handled at the same
> point, rather than separated by a mutex boundary. Because of the
> possibility of a START and HALT/CLEAR running concurrently, it does
> not make sense to split them here.
> 
> With the above patches, maybe it then makes sense to hold the io_mutex
> across the entirety of vfio_ccw_sch_io_todo(). But I'm not completely
> sure that would be acceptable.
> 
> So... Thoughts?

I believe we should address the concurrency, encapsulation and layering
issues in the subchannel/ccw pass-through code (vfio-ccw) by taking a
holistic approach as soon as possible.

I find the current state of art very hard to reason about, and that
adversely  affects my ability to reason about attempts at partial
improvements.

I understand that such a holistic approach needs a lot of work, and we
may have to stop some bleeding first. In the stop the bleeding phase we
can take a pragmatic approach and accept changes that empirically seem to
work towards stopping the bleeding. I.e. if your tests say it's better,
I'm willing to accept that it is better.

I have to admit, I don't understand how synchronization is done in the
vfio-ccw kernel module (in the sense of avoiding data races).

Regarding your patches, I have to admit, I have a hard time figuring out
which one of these (or what combination of them) is supposed to solve
the problem you described above. If I had to guess, I would guess it is
either patch 4, because it has a similar scenario diagram in the
commit message like the one in the problem statement. Is my guess right?

If it is right I don't quite understand the mechanics of the fix,
because what the patch seems to do is changing the content of step 4 in
the above diagram. And I don't see how is change that code
so that it does not "misidentifies the data in the IRB as being
associated with the newly active I/O". Moreover patch 4 seems to rely on
private->state which, AFAIR is still used in a racy fashion.

But if strong empirical evidence shows that it performs better (stops
the bleeding), I think we can go ahead with it.

Regards,
Halil